The present invention relates in general to apparatuses and methods for compensating dead time of switching elements which are used in servo motors or inverters for driving motors, and more particularly, to an apparatus and a method for compensating a dead time of a motor driving controller comprising a current command generating unit generating a current command according to a predetermined algorithm, and a current controller generating a voltage command based on the current command.
Conventionally, if an over-rated current flows into a switching transistor for driving a motor, the switching transistor is damaged. If a pair of switching transistors are on simultaneously, an over-rated current flows into current passages of the two devices. To prevent this flowing, dead time for turning off both of the switching transistors is applied. During the dead time thereof, the current normally flows into a diode by a back electromotive force (EMF) from the motor.
Referring to FIG. 1 which is a driving circuit diagram of a motor having a conventional dead time compensating unit, a conventional motor driving circuit is composed of a motor 3, a converter 1 rectifying an alternating current (AC) power into a direct current (DC) power, an inverter 2 converting the DC power from the converter 1 into a signal and supplying the signal to the motor 3, a current sensor sensing a current applied to the motor 3, a controller 6 having a dead time compensating unit 6a and generating a voltage command based on the current value sensed by the current sensor 5, and a pulse width modulator (hereinafter, xe2x80x9cPWMxe2x80x9d) 7 executing a modulation of a pulse width based on the voltage command from the controller 6 and supplying a control signal to the inverter 2.
With this configuration, the converter 1 rectifies a three-phase AC power through six (6) rectifying diodes (D1) and smoothes it through a smoothing condenser (C1) connected in parallel with the rectifying diodes (D1).
The current sensor 5 senses the current supplied to the motor 3 and supplies the sensed current value to the controller 6. Based on the supplied current value, the controller generates a three-phase voltage command signal (V*). The PWM 7 executes the pulse width modulation based on the voltage command signal (V*) to generate the control signal, and supplies the generated control signal to each gate of the switching devices (T1, T2) of the inverter 2.
The inverter 2 switches the DC voltage applied to each gate of the switching devices (T1, T2) in response to the control signal supplied from the PWM 7, and supplies the switched DC voltage to the motor 3. In the switching devices (T1, T2), diodes (D1, D2) are connected in parallel, to prevent damage thereof by the back EMF from the motor 3.
FIGS. 2a and 2b shows operations of the switching devices (T1, T2) according to directions of the current and flowing of the current, wherein the current value when the current flows into the motor 3 from the inverter 2 is indicated positive (+), and the current value when the current flows into the inverter 2 from the motor 3 is indicated negative (xe2x88x92). While the motor is in dead time, the current flows through the diodes (D1, D2) even if the pair of switching devices (T1, T2) are both off. As shown therein, where the current value is positive, the current flows as if T2 is on. Where the current value is negative, the current flows as if T1 is on.
If the current sensor 5 senses the current applied to the motor 3, the controller 6 determines a polarity of the sensed current value. Where the sensed current value is positive, the controller 6 adds a positive value of voltage compensation as predetermined to the voltage command through the dead time compensating unit 6a. Where the sensed current value is negative, the controller 6 adds a negative value of voltage compensation as predetermined to the voltage command. With these additions, the motor can obtain an effect that there is no dead time.
However, according to the conventional dead time compensating apparatus and method, where the current value is small, a problem occurs at a region where the current value is changed from positive to negative or vise versa, passing through zero point: the range of error due to a non-linearity of the switching devices is magnified, and this results in increasing the difference between the voltage command and the actually applied current and a distortion of current waveforms, thereby increasing heat and noise of the motor.
Accordingly, the present invention has been made keeping in mind the above-described problem, and an object of the present invention is to provide an apparatus and a method for compensating a dead time of a motor, fully considering features of switching devices such as voltage drop due to the switching devices.
This and other objects of the present invention may be achieved by a provision of a method of compensating a dead time of a motor driving device comprising a current command generating unit generating a current command according to a predetermined algorithm, and a current controller generating a voltage command based on the current command, comprising the steps of measuring the voltage command while changing the current command; estimating a stator resistance value of the motor from the measured voltage command value; and compensating the dead time based on the estimated stator resistance value (R).
In the estimating step, a change rate of the voltage command relative to the current command is estimated as the stator resistance value.
Preferably, the compensating step is comprised of the substeps of obtaining a voltage compensation amount by multiplying the estimated stator resistance value by the current command and subtracting the resulting value from the measured voltage command; storing the obtained voltage compensation amount; and generating a voltage command, by subtracting the corresponding voltage compensation amount from the voltage command.
According to another aspect of the present invention, this and other objects may also be achieved by a provision of an apparatus for compensating a dead time of a motor, comprising a current command supplying unit supplying a current command having a variety of values to the motor; a current controller generating a voltage command based on the current command; a voltage command measuring unit measuring the voltage command; a stator resistance estimating unit estimating a stator resistance based on the measured voltage command; and a voltage compensation amount estimating unit estimating a voltage compensation amount to compensate the dead time, based on the estimated stator resistance.
Preferably, the stator resistance estimating unit estimates a change rate of the voltage command relative to the current command as the stator resistance.
Desirably, the voltage compensation amount estimating unit obtains the voltage compensation amount by multiplying the estimated stator resistance value by the current command and subtracting the resulting value from the measured voltage command.
Still preferably, the voltage command measuring unit is comprised of a moving average filter.